Ovens for cooking and baking food have been known and used for thousands of years. Basically, oven types can be categorized in four cooking forms; conduction cooking, convection cooking, infrared radiation cooking and microwave radiation cooking.
There are subtle differences between cooking, and baking. Cooking just requires the heating of the food. Baking of a product from a dough, such as bread, cake, crust, or pastry, requires not only heating of the product throughout but also chemical reactions coupled with driving the water from the dough in a predetermined fashion to achieve the correct consistency of the final product and finally browning the outside. Following a recipe when baking is very important. An attempt to decrease the baking time in a conventional oven by increasing the temperature results in a damaged or destroyed product.
In general, there are problems when one wants to cook or bake foodstuffs with high-quality results in the shortest times. Conduction and convection provide the necessary quality, but both are inherently slow energy transfer methods. Long-wave infrared radiation can provide faster heating rates, but it only heats the surface area of most foodstuffs, leaving the internal heat energy to be transferred by much slower conduction. Microwave radiation heats the foodstuff very quickly in depth, but during baking the loss of water near the surface stops the heating process before any satisfactory browning occurs. Consequently, microwave ovens cannot produce quality baked foodstuffs, such as bread.
Radiant cooking methods can be classified by the manner in which the radiation interacts with the foodstuff molecules. For example, starting with the longest wavelengths for cooking, the microwave region, most of the heating occurs because the radiant energy couples into the bipolar water molecules causing them to rotate. Viscous coupling between water molecules converts this rotational energy into thermal energy, thereby heating the food. Decreasing the wavelength to the long-wave infrared regime, the molecules and their component atoms resonantly absorb the energy in well-defined excitation bands. This is mainly a vibrational energy absorption process. In the short wave infrared region of the spectrum, the main part of the absorption is due to higher frequency coupling to the vibrational modes. In the visible region, the principal absorption mechanism is excitation of the electrons that couple the atoms to form the molecules. These interactions are easily discerned in the visible band of the spectra, where they are identified as "color" absorptions Finally, in the ultraviolet, the wavelength is short enough, and the energy of the radiation is sufficient to actually remove the electrons from their component atoms, thereby creating ionized states and breaking chemical bonds. This short wavelength, while it finds uses in sterilization techniques, probably has little use in foodstuff heating, because it promotes adverse chemical reactions and destroys food molecules.
Lightwave ovens are capable of cooking and baking food products in times much shorter than conventional ovens. This cooking speed is attributable to the range of wavelengths and power levels that are used.
There is no precise definition for the visible, near-visible and infrared ranges of wavelengths because the perceptive ranges of each human eye is different. Scientific definitions of the "visible" light range, however, typically encompass the range of about 0.39 .mu.m to 0.77 .mu.m. The term "near-visible" has been coined for infrared radiation that has wavelengths longer than the visible range, but less than the water absorption cut-off at about 1.35 .mu.m. The term "infrared" refers to wavelengths greater than about 1.35 .mu.m. For the purposes of this disclosure, the visible region includes wavelengths between about 0.39 .mu.m and 0.77 .mu.m, the near-visible region includes wavelengths between about 0.77 .mu.m and 1.35 .mu.m, and the infrared region includes wavelengths greater than about 1.35 .mu.m.
Typically, wavelengths in the visible range (0.39 to 0.77 .mu.m) and the near-visible range (0.77 to 1.35 .mu.m) have fairly deep penetration in most foodstuffs. This range of deep penetration is mainly governed by the absorption properties of water. The characteristic penetration distance for water varies from about 50 meters in the visible to less than about 1 mm at 1.35 microns. Several other factors modify this basic absorption penetration. In the visible region electronic absorption of the food molecules reduces the penetration distance substantially, while scattering in the food product can be a strong factor throughout the region of deep penetration. Measurements show that the typical average penetration distances for light in the visible and near-visible region of the spectrum varies from 2-4 mm for meats to as deep as 10 mm in some baked goods and liquids like non-fat milk.
The region of deep penetration allows the radiant power density that impinges on the food to be increased, because the energy is deposited in a fairly thick region near the surface of the food, and the energy is essentially deposited in a large volume, so that the temperature of the food at the surface does not increase rapidly. Consequently the radiation in the visible and near-visible regions does not contribute greatly to the exterior surface browning.
In the region above about 1.35 .mu.m (infrared region), the penetration distance decreases substantially to fractions of a millimeter, and for certain absorption peaks down to 0.001 mm. The power in this region is absorbed in such a small depth that the temperature rises rapidly, driving the water out and forming a crust. With no water to evaporate and cool the surface the temperature can climb quickly to 300.degree. F. This is the approximate temperature where the set of browning reactions (Maillard reactions) are initiated. As the temperature is rapidly pushed even higher to above 400.degree. F. the point is reached where the surface starts to burn.
It is the balance between the deep penetration wavelengths (0.39 to 1.35 .mu.m) and the shallow penetration wavelengths (1.35 .mu.m and greater) that allows the power density at the surface of the food to be increased in the lightwave oven, to cook the food rapidly with the shorter wavelengths and to brown the food with the longer infrared so that a high-quality product is produced. Conventional ovens do not have the shorter wavelength components of radiant energy. The resulting shallower penetration means that increasing the radiant power in such an oven only heats the food surface faster, prematurely browning the food before its interior gets hot.
It should be noted that the penetration depth is not uniform across the deeply penetrating region of the spectrum. Even though water shows a very deep penetration for visible radiation, i.e., many meters, the electronic absorptions of the food macromolecules generally increase in the visible region. The added effect of scattering near the blue end (0.39 .mu.m) of the visible region reduces the penetration even further. However, there is little real loss in the overall average penetration because very little energy resides in the blue end of the blackbody spectrum.
Conventional ovens operate with radiant power densities as high as about 0.3 W/cm.sup.2 (i.e. at 400.degree. F.). The cooking speeds of conventional ovens cannot be appreciably increased simply by increasing the cooking temperature, because increased cooking temperatures drive water off the food surface and cause browning and searing of the food surface before the food's interior has been brought up to the proper temperature. In contrast, lightwave ovens have been operated from approximately 0.8 to 5 W/cm.sup.2 of visible, near-visible and infrared radiation, which results in greatly enhanced cooking speeds. The lightwave oven energy penetrates deeper into the food than the radiant energy of a conventional oven, thus cooking the food interior faster. Therefore, higher power densities can be used in a lightwave oven to cook food faster with excellent quality. For example, at about 0.7 to 1.3 W/cm.sup.2, the following cooking speeds have been obtained using a lightwave oven:
______________________________________ Food Cook Time ______________________________________ pizza 4 minutes steaks 4 minutes biscuits 7 minutes cookies 11 minutes vegetables (asparagus) 4 minutes ______________________________________
For high-quality cooking and baking, the applicants have found that a good balance ratio between the deeply penetrating and the surface heating portions of the impinging radiant energy is about 50:50, i.e., Power(0.39 to 1.35 .mu.m)/Power(1.35 m and greater).apprxeq.1. Ratios higher than this value can be used, and are useful in cooking especially thick food items, but radiation sources with these high ratios are difficult and expensive to obtain. Fast cooking can be accomplished with a ratio substantially below 1, and it has been shown that enhanced cooking and baking can be achieved with ratios down to about 0.5 for most foods, and lower for thin foods, e.g., pizza and foods with a large portion of water, e.g., meats. Generally the surface power densities must be decreased with decreasing power ratio so that the slower speed of heat conduction can heat the interior of the food before the outside burns. It should be remembered that it is generally the burning of the outside surface that sets the bounds for maximum power density that can be used for cooking. If the power ratio is reduced below about 0.3, the power densities that can be used are comparable with conventional cooking and no speed advantage results.
If blackbody sources are used to supply the radiant power, the power ratio can be translated into effective color temperatures, peak intensities, and visible component percentages. For example, to obtain a power ratio of about 1, it can be calculated that the corresponding blackbody would have a temperature of 3000.degree. K, with a peak intensity at 0.966 .mu.m and with 12% of the radiation in the visible range of 0.39 to 0.77 .mu.m. Tungsten halogen quartz bulbs have spectral characteristics that follow the blackbody radiation curves fairly closely. Commercially available tungsten halogen bulbs have successfully been used with color temperatures as high as 3400.degree. K. Unfortunately, the lifetime of such sources falls dramatically at high color temperatures (at temperatures above 3200.degree. K it is generally less that 100 hours). It has been determined that a good compromise in bulb lifetime and cooking speed can be obtained for tungsten halogen bulbs operated at about 2900-3000.degree. K. As the color temperature of the bulb is reduced and more shallow-penetrating infrared is produced, the cooking and baking speeds are diminished for quality product. For most foods there is a discernible speed advantage down to about 2500.degree. K (peak at about 1.2 .mu.m; visible component of about 5.5%) and for some foods there is an advantage at even lower color temperatures. In the region of 2100.degree. K the speed advantage vanishes for virtually all foods that have been tried.
For rectangular-shaped commercial lightwave ovens using polished, high-purity aluminum reflective walls, it has been determined that about 4 kilowatts of lamp power is necessary for a lightwave oven to have a reasonable cooking speed advantage over a conventional oven. Four kilowatts of lamp power can operate four commercially available tungsten halogen lamps, at a color temperature of about 3000.degree. K, to produce a power density of about 0.6-1.0 W/cm.sup.2 inside the oven cavity. This power density has been considered near the minimum value necessary for the lightwave oven to clearly outperform a conventional oven.
There is a need for a kitchen counter-top lightwave oven that plugs into a standard 120 VAC outlet. However, a typical home kitchen outlet can only supply 15 amps of electrical current, which corresponds to about 1.8 KW of power. This amount of power, which is sufficient to operate only two tungsten halogen lamps at a color temperature of about 2900.degree. K, is well below the 4 KW of lamp power previously deemed sufficient to cook food with speeds and food quality significantly superior to a conventional oven. Two such lamps operating at about 1.8 KW only produce a power density of about 0.3-0.45 W/cm.sup.2 inside the rectangular-shaped oven cavity.